1. Field of the Invention
The present invention relates to a thin-film transistor photosensor.
2. Description of the Related Art
Conventionally, as a photosensor, a transistor type structure having a semiconductor thin film for photoelectric conversion formed under a gate insulating film is known. A thin-film transistor structure using amorphous silicon for a semiconductor thin film for photoelectric conversion, because of the necessity of mounting a photosensor on an insulating substrate, is also well known.
Jpn. Pat. Appln. KOKAI Publication No. 8-213584 discloses such a thin-film transistor type photosensor. The photosensor disclosed in JP 08-213584 has a bottom gate electrode, which is made of a light-shielding conductive material, and which is formed under a bottom gate insulating film formed under a semiconductor thin film for photoelectric conversion that is made of amorphous silicon. In this conventional structure, moreover, a top gate insulating film is formed on the semiconductor thin film, and a top gate electrode made of a light-transmitting conductive material is provided on the top gate insulating film. Still further, source and drain electrodes made of a light-shielding conductive material are provided on a pair of ohmic contact layers made of n-type amorphous silicon on two sides of the upper surface of the semiconductor thin film under the top gate insulating film. In this case, the opposing end faces of the source and drain electrodes are parallel to each other, and the distance between them corresponds to a channel length. Due to this structure of the source and drain electrodes in the photosensor disclosed in JP 08-213584, the end portions of the source and drain electrodes that face each other and are parallel to each other block light that would otherwise obliquely strike the photoelectric conversion semiconductor thin film, in particular. This makes it impossible to ensure sufficient photosensitivity (the ratio light current/dark current).